1. Field of the Invention
The present invention relates in general to a process for forming thick film superconducting coatings on a polycrystalline substrate and, more particularly, to a process for forming a thick film of Bi.sub.a Sr.sub.b Ca.sub.c Cu.sub.d O.sub.x (BSCCO) on a polycrystalline substrate.
2. Description of the Prior Art
It has been determined that the critical temperature of BiSrCaCu.sub.x Og compound is largely determined by the sintering conditions (annealing temperatures and cooling rate), as well as the Cu content. For example, it was experimentally observed that for X=1.5, no sign of superconductivity was observed above 10K. Locquet et al., Solid State Communications, Vol. 66, No. 4, pp. 393-395 (1988). These same tests were conducted by sintering pellets of the preparative compounds, heated to temperatures up to 880.degree. C., followed by slow cooling in air to room temperature to yield randomly oriented polycrystalline bodies having 0 resistance at 72K.
It is known that superconductivity can be obtained with Bi--Sr--Ca--Cu--O systems by first calcining in air the mixture of starting oxides and carbonates at temperatures between 800.degree.-900.degree. C. and then annealing at temperatures up to 870.degree. C. It was found that with higher annealing temperatures, the metallic character of the resistivity is often lost, and superconductivity is deleteriously affected S. A. Shaheen, Solid State Communication, Vol. 66, No. 9, pp. 947-951 (1988).
The superconducting transport properties of high temperature superconducting are highly anisotropic, and the critical currents in oriented films are much higher than those in unaligned materials. It is not known whether highly oriented thick films of BSCCO exhibiting superconductivity at elevated temperatures can be formed on a polycrystalline substrate.
It was believed that a superconductor coating should be applied to single crystal substrates for the purpose of matching the lattice structure of the coating with that of the substrate in order to obtain an aligned coating, i.e., a coating with a high current carrying capacity. Conversely, it was believed that the use of a polycrystalline substrate would inhibit the alignment of a superconductor coating thereon; consequently, the use of polycrystalline substrates was considered to be undesirable. For example, a thick BSSCO coating has been applied to a monocrystalline MgO substrate, but no attempt was made to apply it to a polycrystalline MgO substrate. Y. Akamatsu et al., Jpn. J. Appp. Phys. Letters, 27 L1696-L1698 (1988). In the process described by Akamatsu, a powdered mixture of Bi.sub.2,O.sub.3, CaCO.sub.3, SrCO.sub.3, and CuO are ground together and melted at 1200.degree. C. for 30 minutes without calcination or sintering on an MgO single crystal substrate in air. The melt is then cooled in a furnace at about 40.degree. C./minute to a selected temperature between 700.degree.-900.degree. C. for periods of time from 0 to 2 hours, and then air cooled to room temperature. This procedure often results in the formation of undesired impurities, i.e., CaCuO.sub.x or (CaSr)CuO.sub.x or secondary phases.
It is desirable to have a process for forming thick film superconducting coatings of BSCCO on polycrystalline substrates. This is especially true because coatings made via melt/crystallization techniques are useful in the preparation of devices with complex shapes such as magnetic shields and microchip transmission lines. It is also desirable to provide a process of forming such a superconducting coating without significant amounts of undesirable impurities.